Elimination of nucleation sites in pressure chamber for ink jet systems

ABSTRACT

In the particular embodiments of the invention described in the specification, the pressure chamber for an ink jet system is coated with a smooth, conforming layer of a coating material, such as a xylylene polymer material, which is wettable by the ink used with the system to eliminate nucleation sites in the surfaces forming the walls of the chamber and thereby inhibit formation of bubbles from dissolved air contained in ink within the chamber when the ink is subjected to reduced pressure during operation of the ink jet system.

This application is a continuation-in-part of my copending application,Ser. No. 07/158,656, filed Feb. 22, 1988 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to ink jet systems utilizing pressure chambersand, more particularly, to a new and improved ink jet system having apressure chamber arranged to inhibit formation of air bubbles therein.

In many ink jet systems, ink is supplied through a supply duct to apressure chamber which communicates with an outlet orifice, and ink isejected periodically from the orifice by a rapid contraction of thevolume of the compression chamber as a result of action by anelectromechanical transducer, such as a piezoelectric element. The rapidcontraction is preceded or followed by a correspondingly rapid expansionof the chamber volume. During the expansion portion of the ink dropejection cycle, the pressure of the ink in the pressure chamber isreduced significantly, increasing the tendency of any air dissolved inthe ink within the chamber to form bubbles on the surface of thechamber. Bubbles tend to form in that manner especially at nucleationsites in the chamber such as sharp corners, minute cracks or pits, orforeign particles deposited on the chamber surface, where gases can beretained. Because the presence of gas bubbles within the pressurechamber prevents application of pressure to the ink in the desiredmanner to eject an ink drop of selected volume from the orifice at aselected time, it is important to avoid the formation of such bubbles inthe pressure chamber of an ink jet system.

The Hara et al. U.S. Pat. No. 4,296,421 discloses an ink jet systemusing water-based or oil-based ink in which the pressure chamber and thedischarge orifice are subjected to a treatment to make themwater-repellent or oil-repellent so that they are not wetted by the inkused in the system, thereby making it possible to reduce the energyrequired to eject ink drops from the ink jet head. For this purpose, theorifice plate or the ink jet head is sprayed with a dispersion of Teflonor immersed in a toluene solution of a resin, such as silicone, epoxide,polyurethane, xylylene or the like which is not wetted by the ink usedwith the system.

The patent to Matsuzaki, U.S. Pat. No. 4,725,867, discloses a processfor treating synthetic resin materials forming the ink passageways in anink jet system to make them wettable by the ink used in the system so asto inhibit bubble formation. Since the treatment described in thispatent does not change the surface of the materials, it does noteliminate nucleation sites, such as sharp corners, cracks, pits orforeign particles on the surface.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a newand improved ink jet system having a pressure chamber arranged toinhibit the formation of air bubbles.

Another object of the invention is to provide a method for producing apressure chamber for an ink jet system which is effective to inhibit theformation of air bubbles during ink jet operation.

These and other objects of the invention are attained by providing anink jet system having a pressure chamber connected to an ink jet orificeand communicating with an ink supply duct in which the surface of thepressure chamber is coated with a layer of material providing a smooth,continuous surface conforming to the configuration of the chamber wallswhich is wettable by the ink used in the system. Preferably, the coatingmaterial is an organic substance which can be introduced convenientlyinto the chamber of an assembled ink jet system and form a conformingcoating on the chamber walls which has a low affinity for dirt or solidparticulate material that may be contained in the ink used in thesystem. To assure wetting by the ink used in the system, the coatingshould have a surface energy higher than that of the ink. Forconventional hot melt inks, which have a surface energy of no more than32 dynes per cm., the appropriate coating materials include manypolymeric materials, such as polystyrene, polyvinyl alcohol, epoxies andthe like, and especially preferred coating materials are xylylenepolymer materials.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of the invention will be apparent from areading of the following description in conjunction with theaccompanying drawings in which:

FIG. 1 is a schematic fragmentary view in longitudinal sectionillustrating the arrangement of a pressure chamber and its connectionsto an ink jet orifice and a supply duct in a typical conventional inkjet system; and

FIG. 2 is a view similar to that of FIG. 1, illustrating arepresentative pressure chamber for an ink jet system arranged inaccordance with the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the schematic representation of a typical conventional ink jet systemshown in FIG. 1, an ink jet head is conveniently assembled from a seriesof plate-like elements arranged in sandwich form to produce a compositestructure. Thus, an orifice plate 10 has an ink jet orifice 11 whichcommunicates through aligned apertures 12, 13 and 14, respectively, in amembrane plate 15, a cavity plate 16 and a stiffener plate 17 leading toa pressure chamber 18 formed by an opening in a pressure chamber plate19. The thickness of the pressure chamber plate 19 may be about 3 mils,for example, and the pressure chamber 18 may be about 40 mils wide andabout 375 mils long. One side wall of the pressure chamber 18 isprovided by the stiffener plate 17 and the opposite side wall isprovided by a piezoelectric transducer 20 which moves toward or awayfrom the plate 17 in response to electrical signals as described, forexample, in the Fischbeck et al. U.S. Pat. No. 4,584,590. The orificeplate 10 and the plates 15, 16 and 17 may have thicknesses of from about1 to 10 mils each, and the apertures 12, 13 and 14 may be, for example,about 5 to 10 mils in diameter.

At the other end of the pressure chamber 18, the stiffener plate 17 hasan aperture 21 which may be, for example, about 5 to 10 mils indiameter, leading to a cavity 22 in the cavity plate 16 which isconnected to an ink supply duct (not shown). When the ink jet system isin operation, ink 23 fills the cavity 22, the aperture 21, the pressurechamber 18, the apertures 12, 13 and 14, and part of the orifice 11 inthe orifice plate 10 where a meniscus is formed which normally resistsany flow of ink out of the orifice. At the meniscus, however, the ink incontact with the atmosphere absorbs air and dissolved air will bedistributed through the ink in the apertures 12, 13 and 14 and into theink in the pressure chamber 18.

Thereafter, when the wall of the pressure chamber 18 formed by thepiezoelectric transducer 20 moves away from the stiffener plate 17,expanding the pressure chamber to draw in ink from the cavity 22, theresulting reduction of pressure on the ink in the chamber 18 tends toproduce cavitation as a result of the dissolved air, which can cause airbubbles 24 to form at nucleation sites within the chamber. Suchnucleation sites may be provided by sharp discontinuities, such ascracks, pits or corners formed at the line of contact between adjacentplates, they may also be provided by particulate or other contaminationdeposited on the walls of the pressure chamber. Because of the presenceof such nucleation sites in conventional pressure chambers, there willbe a tendency for bubbles to form in the pressure chamber whenever airdissolved in the ink is subjected to reduced pressure during operationof the ink jet system.

In accordance with the present invention, the tendency during operationof the system is substantially eliminated by providing a coating on thesurface of the pressure chamber of chamber, but fills up or smooths outmicroscopic discontinuities, such as pits, cracks and sharp corners, inthe surface of the pressure chamber walls. A typical arrangementaccording to the invention is shown in FIG. 2 wherein a thin, continuouscoating 25 covers the walls of the chamber 18 and extends into theapertures 12, 13, 14 and 21 as well as the cavity 22. Thus, nucleationsites in the pressure chamber and adjacent regions are eliminated.

To be effective for this purpose, the coating material should provide apinhole-free, mechanically flexible coating having a clean surface whichis wettable by the ink used in the system, i.e., having a surface energyhigher than that of the ink. Any conventional type of ink, such aswater-based ink, oil-based ink or hot melt ink, may be used in thepressure chamber of the invention. If the ink normally has a highersurface energy than that of the coating, it can be reduced to a levelbelow that of the coating by the addition of a conventional surfactant.Preferably, the surface of the coating should also be nonconductiveelectrically.

Also, to assure a smooth, continuous surface on the interior of thepressure chamber which is free of microscopic discontinuities, thesurface coating should preferably be applied after the pressure chamberand its related connections to the ink jet orifice and the ink supplyduct have been assembled. Otherwise, discontinuities may appear, forexample, between the coatings on the surfaces of the separate plateswhich are assembled to form the pressure chamber and related ink ducts.Thus, the material from which the coating is made should preferablycomprise a fluid such as a liquid which may be passed through the ductsand apertures into the pressure chamber to leave a thin, uniform coatingon the surfaces, or a material which can be passed through the system invapor or suspended particulate form to condense or deposit on thesurfaces and coagulate or coalesce into a uniform, smooth coating.

To provide the necessary electrical, mechanical and surface properties,polymer coating materials such as epoxy, urethane and similar materialsare preferred. Especially preferred are the xylylene polymer materials,such as poly(p-xylylene) and poly(chloro-p-xylylene) which can beproduced by vaporizing the dimer form to form a vapor which polymerizesupon condensation to form a uniform conforming thin-film polymer coatinghaving the desired electrical, mechanical and surface properties. Sincethin layers or films of xylylene polymers can be deposited from thevapor phase in a nondirectional manner, the pressure chamber in an inkjet system can be provided with a uniform thin conforming coating ofsuch polymer materials after assembly of the ink jet head by exposingthe ink jet system to the vapor phase of the xylylene material.

Polyxylylene coatings have a surface energy of at least 33 dynes per cm.Other polymeric materials suitable for use with conventional hot meltinks or other inks having a surface energy less than that of the coatingmaterial include polystyrene (33 dynes per cm.), polyvinyl alcohol (37dynes per cm.), epoxy polymers (about 38 dynes per cm.), polymethylmethacrylate and polyvinyl chloride (39 dynes per cm.), polyvinylidenechloride (40 dynes per cm.), polyethylene terephthalate (43 dynes percm.) and polyimides such as polyhexamethylene adipamide which havesurface energies of at least 46 dynes per cm. Inks having a highersurface energy than the coating material, such as certain water-basedinks which may have a surface energy as high as 70 dynes per cm., can beused if a surfactant is added to reduce the surface energy of the ink toa level below that of the coating material. Alternatively, the coatingfor the pressure chamber may be made of a material having a highersurface energy to permit such inks to be used in the system.

To provide a thin, conforming xylylene polymer coating 25 on the wallsof a pressure chamber such as the chamber 18 shown in FIG. 2, the inkjet head assembly consisting of the plates 15, 16, 17, 19 and 20,preferably with the orifice plate 10 removed, is subjected to a reducedpressure such as about 0.1 torr. The dimer form of the desired xylylenematerial, such as dichloro-di-p-xylylene, which is availablecommercially under the name Parylene D, is vaporized at about 250° C. ata pressure of 1 torr and heated to about 600° C. at 0.1 torr to producethe monomer form which is then applied to an ink jet head assemblymaintained at about 25° C. On contact with the surfaces of the ink jetassembly, the monomer condenses and polymerizes to form a continuousthin conforming coating on the surfaces. For other polymer coatingmaterials which do not vaporize and condense in the same manner, anyappropriate conventional application procedure such as spraying ordipping may be used.

If the surface of the pressure chamber is not required to be insulating,any suitable metallic coating material may be used. Clean metalstypically have a surface energy in the range of about 400 to 2000 dynesper cm. Metallic coatings may be applied in any conventional manner suchas by vaporization of the metal and solidification into a continuouslayer on the pressure chamber surfaces. Preferably, the conformingcoating on the surfaces forming the pressure chamber should be fromabout 0.1 to about 5 microns thick and, most preferably, between about0.2 and about 2 microns thick. Since poly(chloro-p-xylylene) is normallydeposited from vapor at a rate of about 0.5 microns per minute at roomtemperature, a 2-micron-thick layer 25 can be coated on the walls of thepressure chamber 18 in about 4 minutes. Poly(p-xylylene) layers formmore slowly and may require considerably more time to attain the samethickness under the same conditions.

With a smooth, continuous, conforming layer of the type described hereincoated on the walls of a pressure chamber which is wettable by the inkused in the system, nucleation sites which lead to formation of bubbleswhen ink containing dissolved air is subjected to reduced pressure aresubstantially eliminated. As a result, ink containing some dissolved aircan be subjected to greater pressure reduction without causing bubbleformation in the pressure chamber, or ink containing an increased amountof dissolved air can be subjected to the same pressure reduction whichwould otherwise produce bubbles in the pressure chamber. Consequently,the improved pressure chamber for an ink jet system according to thepresent invention which effectively inhibits formation of air bubblesovercomes disadvantages of present ink jet systems and permits operationof ink jet systems over a wider range of conditions.

Although the invention has been described herein with reference tospecific embodiments, many modifications and variations therein willreadily occur to those skilled in the art. Accordingly, all suchvariations and modifications are included within the intended scope ofthe invention.

I claim:
 1. A pressure chamber for an ink jet system comprising achamber formed by a plurality of wall segments, a supply of ink in thechamber having a selected surface energy, first aperture means extendingthrough a wall segment and communicating with an ink jet orifice, secondaperture means extending through a wall segment and communicating withan ink supply duct, and a layer of xylylene polymer coating materialforming a smooth, continuous, impermeable coating conforming to theconfiguration of the wall segments of the chamber, the coating beingmechanically wettable by the ink, thereby eliminating nucleation sitesfor bubble formation when ink containing dissolved air within thechamber is subjected to a reduced pressure.
 2. A pressure chamberaccording to claim 1 wherein the coating on the wall segments is betweenabout 0.1 and about 5 microns thick.
 3. A pressure chamber according toclaim 2 wherein the coating on the wall segments is between about 0.2and about 2 microns thick.
 4. A pressure chamber according to claim 1wherein the coating comprises a polymer material.
 5. A pressure chamberaccording to claim 1 wherein the coating comprises a material having asurface energy of at least about 33 dynes per cm. and the surface energyof the ink is less than about 33 dynes per cm.
 6. A pressure chamberaccording to claim 1 wherein the coating comprises poly(p-xylylene). 7.A pressure chamber according to claim 1 wherein the coating comprisespoly(chloro-p-xylylene).
 8. A method for preparing a pressure chamberfor an ink jet system for use with ink having a selected surface energycomprising forming a chamber having a plurality of wall surfaces andhaving a first aperture for communication with an ink jet orifice and asecond aperture for communication with an ink supply duct, andintroducing a xylylene coating material into the chamber so as todeposit a smooth, continuous coating of the material conforming to thewall surfaces of the chamber, the coating being mechanically wettable bythe ink used with the system.
 9. A method according to claim 8 includingthe step of vaporizing a xylylene material and introducing the xylylenevapor into the pressure chamber and depositing a coating comprisingxylylene polymer material on the wall surfaces of the pressure chamber.10. A method according to claim 9 wherein the coating deposited on thechamber wall surfaces comprises poly(chloro-p-xylylene).
 11. A methodaccording to claim 9 wherein the coating deposited on the chamber wallsurfaces comprises poly(p-xylylene).
 12. A method according to claim 8wherein the coating has a surface energy of at least about 33 dynes percm. and the ink to be used with the system has a surface energy of lessthan about 33 dynes per cm.